Bulk pressure in fluid-dynamical simulations of Pb-Pb and p-Pb collisions at the LHC energies

Abstract

State-of-the-art fluid dynamical simulations of relativistic heavy-ion collisions employ initial state models which result in a rather strong radial flow. In order to fit the experimental observables, a non-negligible bulk viscosity of the QGP and/or hadronic matter is required. We examine modern parametrizations of the bulk viscosity to entropy density ratio $\zeta/s$ used in recent fluid dynamical simulations, and explore the relative magnitude of the associated bulk viscous corrections in space-time, for Pb-Pb and p-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV LHC energy with a state-of-the-art initial state provided by TRENTo model. It turns out that, at a typical particlization hypersurface, the effect of bulk viscosity out-competes the one of shear viscosity, and in a significantly large part of the space-time volume, the bulk pressure strongly counteracts the equilibrium pressure, thus the effective pressure approaches zero. The latter potentially challenges the applicability of fluid-dynamical modelling of heavy ion-ion and proton-ion collisions at the LHC energies.